An optical transmitter including a Mach-Zehnder modulator driven by an electric signal is known as an example of an optical transmitter that provides a high-speed data communication (for example, Japanese Laid-open Patent Publication No. 2014-138361 and Document 1). Continuous wave light is input to the Mach-Zehnder modulator. Then, a modulated optical signal is generated by driving the Mach-Zehnder modulator with an electric signal indicating transmission data.
FIG. 1 illustrates an example of an optical transmitter including a Mach-Zehnder modulator. In this example, the optical transmitter includes a Mach-Zehnder modulator 1, a driver 2a, and a driver 2b. A modulation area of the Mach-Zehnder modulator 1 is divided into a plurality of modulation areas. In the example illustrated in FIG. 1, the Mach-Zehnder modulator 1 includes a modulation area 1a and a modulation area 1b. Non-modulated continuous wave light is input to the Mach-Zehnder modulator 1. The driver 2a generates a drive signal (a) from data (a), and the driver 2b generates a drive signal (b) from data (b). Then, the drive signal (a) and the drive signal (b) are respectively applied to the modulation area 1a and the modulation area 1b. This configuration provides a pulse-amplitude modulation (PAM) that transmits the data (a) and the data (b). In the configuration illustrated in FIG. 1, 4-level pulse-amplitude modulation (PAM4) is provided if the length of the modulation area 1b is twice the length of the modulation area 1a (for example, Document 2).
In 4-level pulse-amplitude modulation, for example, the following optical amplitudes A are obtained for a combination of data (a) and data (b).    “a=0, b=0”: “A=0”    “a=1, b=0”: “A=1”    “a=0, b=1”: “A=2”    “a=1, b=1”: “A=3”
It is assumed that, in the optical transmitter illustrated in FIG. 1, the drive signals (a) and (b) are given to the Mach-Zehnder modulator 1 at the same timing. However, light input to the Mach-Zehnder modulator 1 passes through the modulation area 1a and then passes through the modulation area 1b. Thus, the timing at which the input light is modulated by the drive signal (b) in the modulation area 1b is shifted, by a light propagation delay time τ, with respect to the timing at which the input light is modulated by the drive signal (a) in the modulation area 1a. τ depends on the length of the modulation area 1a. The strength to modulate an optical signal depends on the strength of a drive signal. Thus, the waveform of a modulated optical signal output from the Mach-Zehnder modulator 1 will be distorted if the timings at which input light is modulated are different.
This problem may be solved if a drive signal (b) output from the driver 2b is delayed by a time τ with respect to a drive signal (a). Delaying an electric signal given to a Mach-Zehnder modulator is disclosed in, for example, Document 3. Further, an optical transmitter that can operate normally even if a data-transmission speed varies is disclosed in, for example, Japanese Laid-open Patent Publication No. 2003-218790.